1. Field of the Invention
The present invention relates to precipitated silicas which have particularly broad pore size distribution of pores with a pore diameter smaller than that of the maximum of the derivative of the pore volume distribution function, with simultaneously very low microporosity and high rubber activity, to a process for their production and to their use as filler for rubber mixtures.
2. Description of the Related Art
The use of precipitated silicas in elastomer mixtures, e.g. tire tread mixtures, has long been known (EP 0501227). Stringent requirements are placed upon the use of silicas as reinforcing filler in rubber mixtures, for example those used for the production of pneumatic tires and of technical rubber products. The intention is that they can be incorporated and dispersed within the rubber easily and efficiently and that in combination with a coupling reagent, preferably a bifunctional organosilicon compound, they enter into chemical bonding with the rubber, leading to the desired high level of reinforcement of the rubber mixture. The reinforcing property can in particular be associated with high static stress values and a low abrasion value. Factors of decisive importance for the reinforcing property of the silicas are in particular the particle size, surface morphology and surface activity, and the coupling power of the coupling reagent.
The person skilled in the art is also aware that low-molecular-weight compounds, e.g. the bifunctional organosilicon compounds and vulcanization accelerators can undergo physi- and chemisorbtion within the pores of the microporous silica and thus retain only restricted ability to exert their function as rubber coupling agent or vulcanization accelerator for rubber crosslinking.
The person skilled in the art is moreover aware that the coupling reagent, usually a bifunctional organosilicon compound known from S. Wolff, “Chemical Aspects of Rubber Reinforcement by Fillers”, Rubber Chem. Technol. 69, 325 (1996) is intended to modify the rubber-functional surface very homogeneously and quantitatively. The modification method can be precoating of the silica in bulk or in solution/suspension (ex-situ) (U. Görl, R. Panenka, Silanized silicas—A new class of materials for development of new mixtures [in German], Kautsch. Gummi Kunstst. 46, 538 (1993)) or else during the mixing process (in-situ) (H.-D. Luginsland, Processing of Silica/Silane-Filled Tread Compounds, paper No. 34 presented at the ACS Meeting, 4-6 Apr. 2000, Dallas, Tex./USA), in-situ modification being the preferable, and conventional process. In order to ensure rapid and quantitative silanization of the rubber-functional surface, it is therefore necessary to achieve a controlled increase in the concentration at the surface of reactive silanol groups, at which the coupling of the organosilicon compound can take place. The number of silanol groups can be quantified inter alia as Sears number V2 (cf. R. K. Iler, The Chemistry of Silica, John Wiley & Sons (1979)). The interaggregate structure of silicas is usually determined by means of the DBP test (J. Behr, G. Schramm, Über die Bestimmung derÖlzahl a von Kautschukfüllstoffen mit dem Brabender-Plastographen [Determination of oil number of rubber fillers using the Brabender Plastograph] Gummi Asbest Kunstst. 19, 912 (1966)). A high DBP number is needed in order to ensure ideal dispersion of the filler in the rubber. A moisture level >4% is needed in order to ensure rapid and complete silanization of the silica surface with an organosilicon compound (U. Görl, A. Hunsche, A. Müller, H. G. Koban, Investigations into the Silica/Silane Reaction System, Rubber Chem. Technol. 70, 608 (1997)). The moisture level is typically from 4 to 8%.
There are various processes known to the person skilled in the art for the production of silicas. For example, precipitations at constant pH have been described by EP 0937755. DE 10124298 has disclosed silicas precipitated with a constant excess of cations. EP 0755899 A2 and U.S. Pat. No. 4,001,379 have described precipitations at constant alkali number. Silicas for rubber applications are often produced with maintenance of a constant pH of from 7 to 10, see EP 0901986 A1 by way of example.
WO 2003016215 describes silicas which have a pore ratio of V(d5-d50)/V(d5-d100)>0.66. EP 0942029 in turn discloses silicas for elastomer mixtures which have two different types of aggregate.